U.S. Pat. No. 2,357,484, issued to Martin in 1944 discloses that epsilon-aminocapronitrile (ACN) can be converted into epsilon-caprolactam (CL) by contacting water with the ACN in the vapor phase in the presence of a dehydrating catalyst. Martin also described a liquid phase process to produce CL. See U.S. Pat. No. 2,301,964, issued Nov. 17, 1942.
In more recent years, technology has been developed to make inexpensive adiponitrile (ADN) by direct hydrocyanation of butadiene. This discovery has led to a renewed interest in the Martin CL process because the inexpensive ADN can be partially hydrogenated to produce an impure product that comprises ACN. This impure product also contains some byproducts of the hydrogenation reaction, notably tetrahydroazepine and its derivatives (both of which being referred to hereinafter as “THA”).
Some recent patents have expressly taught that the THA and its derivatives must be removed from the impure ACN product before the ACN is converted into CL. See, for example, U.S. Pat. No. 6,169,199, issued Jan. 2, 2001.
Contrary to the suggestions in these patents, it has been found that the impure ACN that is recovered from the partial hydrogen of ADN—that contains greater than 500 ppm THA and its derivatives—can be processed in the vapor phase, as taught by Martin, to make CL without first removing the THA and its derivatives, and that the THA and its derivatives can be removed easily by distillation from the resulting crude CL product.